Multi-variable operations

ABSTRACT

Operation of a multi-variable process involves multidimensional representation of the value (pl-p 12 ) of the process variables (P 1 -P 12 ) according to individual coordinate axes, and an operational envelope (UB,LB) derived from a group of sets of values for the process and quality variables (P 1 -P 12 ,Q 1 -Q 2 ) accumulated from multiple, earlier operations of the process, defines an operating zone for the process and quality variables of the process. If the current value (p 7 ) of any process variable (P 7 ) goes outside the envelope, an envelope (UO,LB) for a different, wider grouping of the stored data-sets is displayed at least for the quality variables (Q 1 -Q 2 ). A series of nested envelopes to provide stepwise enlargement of the operating zone may be provided, but non-nested envelopes can be used where there is clustering of acceptable values of process variables of the stored data-sets. The changes to control variables to bring the values of dependent variables within a best operating range can be determined.

This application is a national stage completion of PCT/GB2004/003768filed Sep. 6, 2004 which claims priority from British Application SerialNo. 0320670.3 filed Sep. 4, 2003.

FIELD OF THE INVENTION

This invention relates to multi-variable operations.

BACKGROUND OF THE INVENTION

The invention is particularly concerned with methods of operating acontrollable multi-variable process and systems for use in the operationof such a process. Methods and systems of this kind are described inGB-A-2 363 647 and GB-A-2 378 527 in which a multi-dimensional displayrepresentation of the variables is derived according to individual,parallel or other spaced coordinate axes. The variables of the processare of two kinds, namely, process-variables, of which the current valuesare generally available during process operation, and quality-variables,of which the values are generally determined later by laboratory orsimilar measurement or analysis. A boundary or envelope for prospectiveoperation of the process is defined within the system of axes inaccordance with sets of values for the process- and quality-variablesaccumulated respectively from previous multiple operations of theprocess. Each previous operation is defined as a respective datapointconsisting of a set of values for the individual process- andquality-variables applicable to that operation.

The multi-dimensional display representation of these earlier-describedmethods and systems is of advantage in the control of the process byidentifying in relation to the defined envelope whether the currentoperating point as represented by the current values of theprocess-variables, lies within the range of past experience, andfacilitates prediction of the likely outcome of the process. It is anobject of the present invention to provide a method of operating acontrollable multi-variable process, and a system for use in theoperation of such a process, of improved form.

SUMMARY OF THE INVENTION

According to respective aspects of the present invention there isprovided a method, and a system, for operating a controllablemulti-variable process, wherein in each aspect a multi-dimensionaldisplay representation of the variables according to individualcoordinate axes is derived, sets of values of process- andquality-variables accumulated respectively from previous multipleoperations of the process are stored, and current values of the process-and quality-variables are indicated on their respective axes in relationselectively to one or other of a plurality of operational envelopeswhich are each derived from at least some of the accumulated sets ofvalues and which define bounds for the variables and differ from oneanother in the bounds defined for at least one of the variables, theparticular one of the envelopes in relation to which the current valuesare indicated being dependent on the current value of at least one ofthe process-variables.

The envelopes may be nested within one another, and in this case theenvelopes may differ from one another in the bounds defined for thequality-variables. More especially, the particular one of the envelopesin relation to which the current values are indicated is changed whenthe current value of any one of the process-variables moves outsidebounds defined for it by that envelope, the change being to another ofthe envelopes for which the bounds applicable to the quality-variablesare wider.

The known methods and systems described in GB-A-2 363 647 and GB-A-2 378527 involve the calculation and use of either of two envelopes. A firstof these envelopes, termed the BOZ, representing the best operating zoneof the process, is calculated from a selected, limited group of the setsof accumulated data, whereas the second envelope, a more general, outerenvelope, is calculated from all the accumulated sets of data. Thislarger group includes the limited group used for the BOZ, and so theenvelope of the BOZ lies wholly within the outer envelope. The twoenvelopes are used for different purposes in the known methods andsystems, the outer envelope for the purpose of revealing the regionwithin which operation of the process may take place within the boundsof past experience. The envelope of the BOZ, on the other hand,establishes the ranges of the individual process-variables that areapplicable to realisation of ‘good’ results and provides a prediction ofthe ranges of the quality-variables that can be expected to be achieved.However, this prediction remains valid only while the values of all theprocess-variables remain within their respective BOZ-envelope limits.

The present invention enables a more flexible approach in that detectionof the condition in which any one or more of the process-variables liesoutside its respective BOZ-envelope limit, may bring about a change toanother envelope for which the bounds applicable to theprocess-variables remain the same, but those for the quality-variablesare those applicable to the outer envelope.

In accordance with a further feature of the present invention, aplurality of non-nested envelopes may be used, and in this case thecurrent values may be indicated in relation to one or other of theenvelopes in dependence upon whether the current value of an individualprocess-variable is within a respective part of its range of values.This may be used to provide different BOZ-envelopes for different partsof the range of values of one or more process-variables, so that a moreaccurate prediction of quality-variable values can be made.

The invention may be applied to optimisation of the process beyond whatis directly achievable using the BOZ-envelope. To this end, the currentvalue of one or more of the process-variables may be changed to bringabout a change of envelope by which the lower limiting-bound indicatedby it in relation to one or more of the quality-variables is increasedwhereas the upper limiting-bound in relation to another of thequality-variables is decreased. The advantage of this technique is thatit enables the values of ‘good’ quality-variables to be increased at thesame time as decreasing the values of ‘bad’ quality-variables, in a waymore readily realisable than when operating solely within aBOZ-envelope.

According to features of the present invention there is provided amethod, and a system, for operating a controllable multi-variableprocess, wherein a multi-dimensional display representation of thevariables according to individual coordinate axes is derived, sets ofvalues of process- and quality-variables accumulated respectively fromprevious multiple operations of the process are stored, first and secondoperational envelopes for the process- and quality-variables arecalculated, the first and second envelopes being related respectively tobounds for the process- and quality-variables of the process and beingderived from the accumulated sets of values to the extent that the firstenvelope is derived from a selected, limited group of the sets and thesecond envelope is derived from a larger group that includes saidlimited group, indicating current values of the process- andquality-variables on their respective axes, detecting the condition inwhich any of the current values of the process-variables lie outside thefirst envelope, and including representation in the displayrepresentation of the first or the second operational envelope at leastinsofar as it relates to the quality-variables, in dependence uponwhether or not, respectively, said condition is detected.

Additionally, there is provided according to further features of thepresent invention, a method, and a system, for operating a controllablemulti-variable process, wherein a multi-dimensional displayrepresentation of the variables according to individual coordinate axesis derived, sets of values of process- and quality-variables accumulatedrespectively from previous multiple operations of the process arestored, and current values of the process- and quality-variables areindicated on their respective axes in relation to an operationalenvelope defining bounds for the process- and quality variables, theenvelope being calculated from one or another of different parts of aselected group of the accumulated sets of values according to thecurrent value of at least one of the process-variables, the part of thegroup from which the envelope is calculated comprising sets of the groupwhich, in distinction to the sets of the other part or parts, havevalues for said one process-variable clustered on the current valuethereof.

BRIEF DESCRIPTION OF THE DRAWINGS

A method and system according to the present invention will now bedescribed, by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic representation of a system according to theinvention in the context of collection and utilisation of data derivedfrom operation of a multi-variable processing plant;

FIG. 2 is illustrative of a display representation in multi-dimensionalspace defined by parallel coordinate axes, derived during operation ofthe system of FIG. 1 according to the invention;

FIG. 3 shows part of a multi-dimensional display of historical dataaccumulated from multiple process-operations of the plant of FIG. 1; and

FIG. 4 is illustrative of a further form of display representation inthe multi-dimensional space format defined by parallel coordinate axes,derived during operation of the system of FIG. 1 according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The example of method and system to be described is related to thecontrol of operation of a multi-variable process carried out by asimple, notional processing plant. Details of the plant and its purposeare not of consequence, and indeed the method and system of theinvention are related more specifically to operation of the plant as anexample of a multi-variable process rather than to the purpose of theprocess performed, being applicable in the generality to any situationinvolving a multi-variable process. In the context of initialdescription of the present specific example, however, there are fourteenvariables involved in plant-operation, and of these, twelve areprocess-variables to the extent that their values determine the outcomeof the process. The remaining two variables are quality-variables in thesense that their values define, or more especially are defined by, thatoutcome.

Referring to FIG. 1, the plant 1 has an input 2 and an output 3 betweenwhich there are, for example, a multiplicity of processing stages 4. Theprocessing within each stage 4 is carried out in accordance with one ormore variables that, in this example, are regulated by controllers 5.The values of these variables for each operation or ‘run’ of the processare communicated to a data collection unit 6 to be accumulated in astore 7. The term ‘run’ in this context may refer to a discreteoperation of the process, but it may also refer to what applies at adiscrete point in time within continuous operation.

The outcome at the output 3 of each run of the process, is submitted toa unit 8 for analysis in respect of its quality as determined accordingto two variables. The values of these two quality-variables areaccumulated in a store 9, so that each run of the process and itsoutcome is defined by an accumulated set of fourteen values, twelve inthe store 7 and three in the store 9, for the fourteen variablesrespectively.

As the process is run again and again, a multiplicity of different setsof fourteen values are accumulated, and from these a selection is madeto provide a historical record in the stores 7 and 9 of successive runsrepresenting satisfactory operation of the process. This record is usedin the method of the present invention as a basis for selection of thevalues of the various variables appropriate to achieving a particularoutcome. More especially, the fourteen values of each individual set,twelve in the store 7 and two in the store 9, are brought together in amerge unit 10 and each scaled to the range 0 to 1. The scaled values arethen processed in a unit 11 to plot them in a multi-dimensional displayrepresentation provided by an electronic display unit 12.

The unit 11 operates generally as described in GB-A-2 363 647 and GB-A-2378 527 to calculate a BOZ envelope representing the best operating zoneof the process from a selected, limited group of the sets of dataaccumulated in the stores 7 and 9, and also a more general, outerenvelope calculated from a larger group of the accumulated data. Thelarger group used for the outer envelope includes the limited group usedfor the BOZ envelope, the two envelopes being calculated in this caseusing convex hulls between pairs of the variables and being displayed bythe unit 12 in a multi-dimensional space defined by parallel axes.

FIG. 2 is illustrative of the display representation of the twelveprocess-variables P1-P12 and two quality-variables Q1-Q2 of the process.The fourteen variables P1-P12 and Q1-Q2 are all scaled to the range 0 to1 and allocated to respective axes of a set of fourteen parallel axes.The values p1-p12 of the process-variables P1-P12 for the currentoperating point of the process are plotted on their respective axes.

The current operating point (p1-p12) in the case represented in FIG. 2,has departed from the BOZ, that is to say, from the zone defined betweenthe two polygonal lines which demarcate respectively the upper and lowerboundaries UB and LB of the best operating zone. In the illustratedexample, the departure arises from the condition in which the plottedvalue of just one of the process-variables P1-P12, namely the value p7of the variable P7, is outside the BOZ envelope. This alarm condition isindicated in the display representation by a caret C emphasising thelocation of the violated limit on the axis of variable P7.

Until the alarm condition arose, the full BOZ envelope for the datapointwas shown, that is to say, for the current condition of the processextending throughout the full range of the fourteen axes for thevariables P1-P12 and Q1-Q2. As soon as the alarm condition arises,however, the prediction is no longer valid and the displayrepresentation is changed automatically by the system to replace the BOZenvelope by a new envelope as shown in FIG. 2. More particularly, theranges of the quality-variables Q1 and Q2 are indicated in the newenvelope by upper and lower boundary lines UO and LO that are the sameas those of the outer envelope for the variables Q1-Q2. The same upperand lower boundaries UB and LB of the BOZ envelope, however, arereproduced in the new envelope for the process-variables P1-P12, and arejoined up with the upper and lower boundaries UO and LO, respectively,to provide transition between the axes of the variables P12 and Q1. Theupper and lower boundary lines UO and LO extending between the axes ofthe variables P12 and Q2, are represented in a different colour from thedemarcation lines UB and LB so as to show that the limits now indicatedfor the quality-variables Q1-Q2 have been calculated using the outerenvelope.

The limits on quality-variables predicted using the outer envelope,while valid, are often very wide. It would often be useful to havetighter predictions on the quality-variables when the operating point isoutside the BOZ. For this purpose a set of nested envelopes may be used,each of which is the envelope of a set of datapoints chosen withsuccessively more restrictive criteria; the envelope with the mostrestrictive criteria is the BOZ envelope. If there are, for example,four envelopes E1-E4 in all, then the outer envelope E1 encloses a firstintermediate envelope E2, which encloses a second intermediate envelopeE3, and this second, intermediate envelope encloses the BOZ envelope E4.Then, if an operating point lies outside the BOZ envelope E4 but withinintermediate envelope E3, intermediate envelope E3 is used to calculatethe limits on the quality-variables. If the operating point lies outsideintermediate envelope E3 but inside intermediate envelope E2,intermediate envelope E2 is used to calculate the limits. Otherwise, theouter envelope E1 is used.

The facility by which the boundaries for the quality-variables arechanged in dependence upon the location of the operating point, fromthat of the BOZ to a wider envelope, whether this is directly to theouter envelope as illustrated in FIG. 2, or to, or through, one or moreintermediate stages (such as the envelopes E2 and E3), is of assistanceto the process-operator. More particularly, it maintains validindication of the limits applicable to the quality-variables of theproduct currently being made, even when the process has deviated outsidethe BOZ.

Although in the case of the method using multiple envelopes describedabove, the envelopes are nested one within the other, there arecircumstances where non-nested envelopes may be utilised with advantage.These circumstances are illustrated by FIG. 3 which shows part of amonitor display of historical datapoints accumulated from multipleprocess-operations.

Referring to FIG. 3, the accumulated data is displayed plotted against aset of parallel axes; only the part of the display between the axes ofvariables Px, Py and Pz. is represented in FIG. 3. This part of thedisplay indicates that the process has two distinct modes of ‘good’operation, namely, modes Py_High and Py_Low in which the values of theprocess-variable Py are clustered at the top and bottom, respectively,of its range, with an empty space S in between.

Since it is possible to make good product in either of the modes Py_Highand Py_Low, a single BOZ for the whole set of datapoints covered, wouldinclude the empty space S. This may be avoided by defining two BOZs, sayBOZ_Py_High and BOZ_Py_Low, in which the criterion that the value of theprocess-variable Py is in the top cluster-zone or the bottomcluster-zone of values respectively, is combined with the selectioncriteria for ‘good’ operation. The appropriate BOZ is used to calculateoperating limits depending on which part of its range the value ofvariable Py lies in at each time step.

The use of a plurality of separate BOZs has advantage also where theaccumulated, historical datapoints are distributed all over the range ofa variable, but the BOZ contains distinct clusters that are separated bysub-ranges where ‘good’ product is never made. Two or more BOZs may beused in these circumstances with the advantage that a more-accurateindication of operating limits of the process-variables and of predictedlimits on quality-variables is provided than if one overall BOZ wereused.

GB-A-2 378 527 contains description of a method and system forcalculating and displaying suggested changes in the values of thecontrol or manipulated variables (namely, of the process-variableshaving values that can be changed directly by the operator) that willclear current alarms. This method rectifies alarms on dependent ornon-manipulated variables, namely, on process-variables which havevalues that cannot be changed directly by the operator, but whose valueschange in dependence upon change of control- or manipulated-variables.In this regard, the values of the control-variables are moved in such away that the limits due to the BOZ on the alarm-dependent variables aremoved out to the current values of those variables, thus putting thosedependent-variables inside the BOZ. This is only possible if thealarm-dependent variable is within its BOZ range, that is to say, iswithin the range from minimum to maximum values that ever occur in theBOZ (the upper and lower horizontal axes in the display of FIG. 2represent the top and bottom of the BOZ range for each variable). If thealarm-dependent variable is not within this range, the outer envelopecan be used to generate alarm rectification advice. As the outerenvelope represents the full range of process behaviour, it can be usedto calculate the effect of the changes in the control-variables on thedependent-variables.

If the current value of a dependent-variable is below the bottom of itsBOZ range, the required action is to manipulate the control-variables sothat the lower limit on that dependent-variable due to the outerenvelope, is above the bottom of its BOZ range. This means that in thereal process, the change in the control-variables will cause the valueof the dependent-variable to increase to at least its new lower limitdue to the outer envelope.

On the other hand, if a dependent-variable is currently above the top ofits BOZ range the control-variables must be manipulated so that theupper limit on that dependent-variable due to the outer envelope isbelow the top of its BOZ range. This means that in the real process thechange in the control-variables will cause the value of thedependent-variable to decrease to, at most, its new upper limit due tothe outer envelope; the time for this to be completed depends on theprocess. If when this achieved there are any alarms due to the BOZ, itwill be possible to rectify them by the method described in GB-A-2 378527.

The algorithm for calculating the necessary moves to bring the requiredlimit on a dependent-variable to within its BOZ range, is quite similarto that described in GB-A-2 378 527 for calculating the best set ofmoves of manipulated-variables for rectifying alarms. The convex hullsinvolved in the calculations in the present case are those that make upthe outer envelope, and the objective function to be minimised,equivalent to the ‘total infeasibility’ referred to in GB-A-2 378 527,is:

[the sum of (Bottom of BOZ Range minus Lower Limit) over alldependent-variables whose current value is below the bottom of their BOZrange] plus [the sum of (Upper Limit minus Top of BOZ Range) over alldependent-variables whose current value is above the top of their BOZrange].

For each such dependent-variable, the coefficient of the relevant limitwith respect to each control-variable can be calculated; for example,using the calculation described in GB-A-2 378 527 with regard to thecoefficients of total infeasibility.

The variable with the greatest ‘good’ effect is found, subject to thecondition that the relevant limit is not to be made worse for any suchdependent-variable; that is to say, a move that improves the totalobjective is not allowed if it reduces the lower limit due to the outerenvelope on any dependent-variable that is below the bottom of its BOZrange, or increases the upper limit on one that is above the top of itsBOZ range. Nor must any dependent-variable currently within the BOZ beforced out of its BOZ range by pushing its lower limit due to the outerenvelope above the top of its BOZ range or its upper limit below thebottom.

The coefficients of the BOZ limits on all dependent-variables withrespect to the control-variables are also needed, as no new alarms mustbe created by pushing the BOZ limits on a variable inside its currentvalue. The other limit on moves of control-variables is the range ofvalidity of the coefficients of the objective with respect to the valuesof the control-variables.

The control-variable with greatest effect is selected and moved to themost restrictive limit on its movement. This is repeated until therelevant limits on all such dependent-variables due to the outerenvelope are inside their BOZ range, or no more moves are possible.

The advantage of this feature to process operation is that it providesadvice to the process operator on how to bring the operating point backinto the best operating zone even when some non-manipulated variableshave values outside their full ranges in the BOZ.

Whenever advice to the operator is generated, it would be possible to‘close the loop’ by feeding the recommended new values of thecontrol-variables to the on-line control system as new setpoints. Theonly difference between advisory and closed-loop control is operatorconfidence.

The outer envelope may be used to avoid moves of the control-variablesthat will have an adverse effect on the future values of non-manipulatedvariables. In this regard, use is made of the same calculation as thatusing the outer envelope for alarm rectification, to calculate theeffect on the future values of dependent-variables of a move of acontrol-variable. In this case the calculated effect is used to ensurethat a move of a control-variable to rectify alarms is not made if itwill push the value of a dependent-variable which is currently outsideits BOZ limits, further in the wrong direction. The method takes eachdependent-variable currently in alarm and uses the outer envelope tocalculate the effect on the future value of that dependent-variable ofmoving the value of each control-variable up or down. Thus, ifdependent-variable Pq is currently below its lower BOZ limit, andincreasing the value of control-variable Pr would decrease the futurevalue of Pq, and decreasing the value of Pr would increase the futurevalue of Pq, then Pr is “OK to Move Down” but not “OK to Move Up”. These“OK to Move” properties are used to constrain the choice of moves ofcontrol-variables to rectify alarms, when these moves are calculated asdescribed in GB-A-2 378 527.

A variable that is normally a control-variable may not be directlycontrollable once it goes outside certain limits. In that case it istreated as a dependent-variable and moves of the other control-variablesare calculated using the outer envelope to push its future value backinto its controllable range.

When the operating point is within the BOZ, the limits on thequality-variables are calculated using the BOZ envelope. Being insidethe BOZ means that the values of the quality-variables are alreadysatisfactory, but there may be some quality-variables that it isdesirable to maximise or minimise within the BOZ. For example, the BOZdataset might have been selected by requiring quality-variables on‘good’ qualities such as those of yield, efficiency or purity, to beabove specified minimum values, but it would still be advantageous toincrease them above this. Similarly, it might be desirable to drive somequality-variables, nominally ‘bad’ qualities such as measures ofemissions, as low as possible. As the methods described here calculatelimits, not values, optimisation is achieved by maximising the lowerlimits due to the BOZ on the ‘good’ qualities and minimising the upperlimits due to the BOZ on the ‘bad’ qualities. In this case theobjective, which is to be maximised, is:

[(the sum of the lower limits on ‘good’ qualities) minus (the sum of theupper limits on ‘bad’ qualities)].

If it is appropriate, different weightings may be given to differentqualities. As before, the coefficients of the objective with respect toeach control-variable are calculated. The constraints applicable in thisare that no individual quality within the objective may have itsrelevant limit made worse, and no alarms may be created. Thecontrol-variable with the greatest coefficient is moved as far as theconstraints and the validity of the coefficients will allow, and this isrepeated until no more improvement is possible.

The advantage of this optimisation feature is that the values ofquality-variables can be further improved beyond the improvement alreadyachieved by operating within the BOZ. An example of the displayrepresentation provided using this feature is shown in FIG. 4 whichshows the changed display representation following optimisation.

Referring to FIG. 4, the upper and lower bounds UB and LB respectively,of the envelope that applied before the optimisation process are shownin chain-dotted line, whereas the bounds UB′ and LB′ of the enveloperesulting from it, are shown in full line. In this example, optimisationis brought about by a decrease in the control-variable P8 from value18.100 to value 18.033 and a decrease in the control-variable P9 fromvalue 1983.0 to value 1974.9. The result is an increase in the lowerlimit of the quality-variable Q2 from q21 to q22 and an increase in itsupper limit from q23 to q24.

In the case where the current operating point is outside the BOZ andmoves of the control-variables to bring the process back into the BOZcannot be found, it is still possible to find moves which will move thepredicted limits on the quality-variables towards their BOZ ranges(which normally correspond to specification). The calculation of theselimits moves is the same as that described above for optimisation usingthe BOZ, except that the outer envelope, or an intermediate envelopewhich includes the current operating point, is used. For eachquality-variable, a lower limit due to this envelope which is lower thanthe bottom of that quality-variable's BOZ range is to be increased, andan upper limit due to this envelope which is greater than the top of itsBOZ range is to be decreased. The set of moves of control-variableswhich has the best overall effect on the limits on the quality-variablesdue to the envelope used is calculated. In this way it is sometimespossible to improve the product qualities even when it is notimmediately possible to bring the process back into its best operatingzone.

Although the invention has been described above in the context of amulti-dimensional display representation using parallel axes, the axesmay instead, for example, be radial axes of a circular plot such asdescribed in GB-A-2 378 527.

1. A method for operating a controllable multi-variable process, whereina multi-dimensional display representation of the variables according toindividual coordinate axes is derived, sets of values of process- andquality-variables accumulated respectively from previous multipleoperations of the process are stored, and current values of the process-and quality-variables are indicated on their respective axes in relationselectively to one or other of a plurality of operational envelopeswhich are each derived from at least some of the accumulated sets ofvalues and which define bounds for the variables and differ from oneanother in the bounds defined for at least one of the variables, theparticular one of the envelopes in relation to which the current valuesare indicated being dependent on the current value of at least one ofthe process-variables.
 2. A method according to claim 1 wherein theenvelopes are nested within one another.
 3. A method according to claim2 wherein the envelopes differ from one another in the bounds definedfor the quality-variables.
 4. A method according to claim 3 wherein theparticular one of the envelopes in relation to which the current valuesare indicated is changed when the current value of any one of theprocess-variables moves outside bounds defined for it by that envelope,the change being to another of the envelopes for which the boundsapplicable to the quality-variables are wider.
 5. A method according toclaim 1 wherein the envelopes are not nested within one another.
 6. Amethod according to claim 5 wherein the current values are indicated inrelation to one or other of the envelopes in dependence upon whether thecurrent value of an individual process-variable is within a respectivepart of its range of values.
 7. A method according to claim 1 whereinthe current value of one or more of the process-variables is changed tobring about a change of envelope by which the lower limiting-boundindicated by it in relation to one or more of the quality-variables isincreased whereas the upper limiting-bound indicated by it in relationto another of the quality-variables is decreased.
 8. A method foroperating a controllable multi-variable process, wherein amulti-dimensional display representation of the variables according toindividual coordinate axes is derived, sets of values of process- andquality-variables accumulated respectively from previous multipleoperations of the process are stored, first and second operationalenvelopes for the process- and quality-variables are calculated, thefirst and second envelopes being related respectively to bounds for theprocess- and quality-variables of the process and being derived from theaccumulated sets of values to the extent that the first envelope isderived from a selected, limited group of the sets and the secondenvelope is derived from a larger group that includes said limitedgroup, indicating current values of the process- and quality-variableson their respective axes, detecting the condition in which any of thecurrent values of the process-variables lie outside the first envelope,and including representation in the display representation of the firstor the second operational envelope at least insofar as it relates to thequality-variables, in dependence upon whether or not, respectively, saidcondition is detected.
 9. A method for operating a controllablemulti-variable process, wherein a multi-dimensional displayrepresentation of the variables according to individual coordinate axesis derived, sets of values of process- and quality-variables accumulatedrespectively from previous multiple operations of the process arestored, and current values of the process- and quality-variables areindicated on their respective axes in relation to an operationalenvelope defining bounds for the process- and quality variables, theenvelope being calculated from one or another of different parts of aselected group of the accumulated sets of values according to thecurrent value of at least one of the process-variables, the part of thegroup from which the envelope is calculated comprising sets of the groupwhich, in distinction to the sets of the other part or parts, havevalues for said one process-variable clustered on the current valuethereof.
 10. A system for operating a controllable multi-variableprocess, wherein a multi-dimensional display representation of thevariables according to individual coordinate axes is derived, sets ofvalues of process- and quality-variables accumulated respectively fromprevious multiple operations of the process are stored, and currentvalues of the process- and quality-variables are indicated on theirrespective axes in relation selectively to one or other of a pluralityof operational envelopes which are each derived from at least some ofthe accumulated sets of values and which define bounds for the variablesand differ from one another in the bounds defined for at least one ofthe variables, the particular one of the envelopes in relation to whichthe current values are indicated being dependent on the current value ofat least one of the process-variables.
 11. A system according to claim10 wherein the envelopes are nested within one another.
 12. A systemaccording to claim 11 wherein the envelopes differ from one another inthe bounds defined for the quality-variables.
 13. A system according toclaim 12 wherein the particular one of the envelopes in relation towhich the current values are indicated is changed when the current valueof any one of the process-variables moves outside bounds defined for itby that envelope, the change being to another of the envelopes for whichthe bounds applicable to the quality-variables are wider.
 14. A systemaccording to claim 10 wherein the envelopes are not nested within oneanother.
 15. A system according to claim 14 wherein the current valuesare indicated in relation to one or other of the envelopes in dependenceupon whether the current value of an individual process-variable iswithin a respective cluster-zone of its range of values.
 16. A systemaccording to claim 10 wherein the current value of one or more of theprocess-variables is changed to bring about a change of envelope bywhich the lower limiting-bound indicated by it in relation to one ormore of the quality-variables is increased whereas the upperlimiting-bound indicated by it in relation to another of thequality-variables is decreased.
 17. A system for operating acontrollable multi-variable process, wherein a multi-dimensional displayrepresentation of the variables according to individual coordinate axesis derived, sets of values of process- and quality-variables accumulatedrespectively from previous multiple operations of the process arestored, first and second operational envelopes for the process- andquality-variables are calculated, the first and second envelopes beingrelated respectively to bounds for the process- and quality-variables ofthe process and being derived from the accumulated sets of values to theextent that the first envelope is derived from a selected, limited groupof the sets and the second envelope is derived from a larger group thatincludes said limited group, indicating current values of the process-and quality-variables on their respective axes, detecting the conditionin which any of the current values of the process-variables lie outsidethe first envelope, and including representation in the displayrepresentation of the first or the second operational envelope at leastinsofar as it relates to the quality-variables, in dependence uponwhether or not, respectively, said condition is detected.
 18. A systemfor operating a controllable multi-variable process, wherein amulti-dimensional display representation of the variables according toindividual coordinate axes is derived, sets of values of process- andquality-variables accumulated respectively from previous multipleoperations of the process are stored, and current values of the process-and quality-variables are indicated on their respective axes in relationto an operational envelope defining bounds for the process- and qualityvariables, the envelope being calculated from one or another ofdifferent parts of a selected group of the accumulated sets of valuesaccording to the current value of at least one of the process-variables,the part of the group from which the envelope is calculated comprisingsets of the group which, in distinction to the sets of the other part orparts, have values for said one process-variable clustered on thecurrent value thereof.